Water Shielding in the Terrestrial Planet-forming Zone: Implication for Inner Disk Organics

Within protoplanetary disks, the inner 2 to 3 au is a critical location where much of the process of planet formation is believed to occur, referred to as the planet-forming zone (e.g., Pierrehumbert & Gaidos 2011; Morbidelli et al. 2012; Raymond et al. 2014; Morbidelli & Raymond 2016). Most directly, this region corresponds to radii at which terrestrial planets are formed (Mulders et al. 2015; Madhusudhan et al. 2021). Observations have shown that many stars are expected to have a planet within 1 au (Johnson et al. 2010; Mulders et al. 2018). The inner 1 au is inside or contains the H₂O ice line and water plays a large role in the evolution of life (e.g., Brown et al. 2013; Cockell et al. 2016; Lingam & Loeb 2019). Furthermore, inside the water-ice line, the elemental C/O ratio is predicted to be ∼stellar, which becomes inherited by giant planets (Öberg et al. 2011; Ida et al. 2019; Öberg et al. 2021). Thus, the location of formation impacts chemical composition (e.g., Lahuis et al. 2006; Öberg et al. 2011; Pontoppidan et al. 2011; Walsh et al. 2012; Madhusudhan 2019).

The inner disk is dust rich, leading to high optical depths. This makes it difficult to determine the chemical content of the disk midplane. However, the radiation from the star, which is in close proximity, leads to a heated disk surface (i.e., T_gas > T_dust), which produces a rich spectrum of emission from volatile molecules, particularly at infrared wavelengths (e.g., Carr & Najita 2008; Brown et al. 2013). Water vibrational lines at 3 μm, HCN vibrational at 3.3 μm, and CO vibrational at 4.7 μm have been observed with both Keck Near Infrared Spectrograph (NIRSPEC) and VLT-CRIRES (e.g., Salyk et al. 2011a; Brown et al. 2013; Mandell et al. 2013; Carr et al. 2018). Further lines of H₂O, OH, HCN, C₂H₂, and CO₂ have been detected by the Spitzer Space Telescope Infrared Spectrograph (IRS), which ranges from 10 to 37 μm (e.g., Carr & Najita 2008; Salyk et al. 2008, 2011b; Pontoppidan et al. 2010). All of these lines are thought to originate in the inner 2 to 3 au, which allows us to trace the gas composition. These observations show that diverse chemistry is present in the inner planet-forming zone.

Najita et al. (2013, 2018) investigate the ratio of HCN/H₂O line flux and find it is related to disk dust mass. They argue that this relation is due to the formation of planetesimals that decouple from the dust and lock up water in distant regions. This effectively increases the C/O ratio in the inner disk, directly impacting the HCN/H₂O line flux. Banzatti et al. (2020) explore the same data set and find that the strongest relation is an anticorrelation between ${L}_{{{\rm{H}}}_{2}{\rm{O}}}$ and R_dust. Instead of an elevated C/O, they propose that the inner disk is fed by drifting pebbles, where large disks are a sign of little drift and small disks are a sign of substantial drift. Because these pebbles are water-ice rich, a high drift rate will enhance the inner disk oxygen content when the water ice sublimates. To distinguish these scenarios, we need to understand the C/O ratio of inner disk gas.

One way to determine the inner disk chemical content and C/O ratio is to use detailed thermochemical models. Models such as Dust and LInes (DALI) (Bruderer et al. 2012; Bruderer 2013), RAC2D (Du & Bergin 2014), and Protoplanetary Disk Model (ProDiMo) (Woitke et al. 2009) solve for both the gas thermal physics and the chemical equilibrium, given stellar parameters, the dust properties and mass distribution, and an overall gas-to-dust mass ratio. Based on these models, Woitke et al. (2018) and Anderson et al. (2021) find that altering the C/O ratio changes the predicted emission of molecules arising from inner disk gas. In their models, they have matched emission from multiple organics but underproduced C₂H₂ unless an elevated C/O ratio is invoked.

Bethell & Bergin (2009) showed that strong formation rates of water vapor in hot (>400 K) surface gas can compete with ultraviolet (UV) photodestruction, allowing water to self-shield. Because water has a broad UV photoabsorption cross section (Yoshino et al. 1996), this can also shield other molecules downstream, a process called water UV shielding. Bosman et al. (2022a, hereafter Paper I), Calahan et al. (2022, hereafter Paper II), and Bosman et al. (2022b, hereafter Paper III) have shown that water UV shielding lowers the UV flux deeper into the disk and is important in understanding H₂O, ${{\rm{H}}}_{2}^{18}$ O, and CO₂ emission.

Additional species beyond H₂O, such as H₂, CO, C i, H i, and N₂, are also abundant in the surface layers to potentially impact the UV field. H₂, CO, atomic carbon, and N₂ all absorb wavelengths less than 110 nm, while most other species dissociate at wavelengths >110 nm and most of the UV photons are also in this wavelength range (e.g., Herczeg et al. 2004; Heays et al. 2017). Thus, while the UV attenuation of these species greatly impacts each other, they do not greatly impact the dissociation of other species, in contrast to H₂O. Shielding by atomic H, specifically scattering of Lyα, does not seem to have a big impact on the chemistry of the inner disk (Paper II).

This naturally raises the question as to whether water UV shielding affects the rest of the chemistry in the inner disk. This is what we investigate in this paper: the impact of water UV shielding on chemistry in the inner disk, with the goal of reconciling current models with observations and making predictions for observations with the James Webb Space Telescope.

2.1. Model Setup

2.2. Determination of the Emitting Layer

Figure 1 shows the abundance in the emitting layer for two models, with and without water UV shielding (water shielding and standard, respectively). For H₂O, the predicted abundance remains relatively similar at radii within the CO₂ ice line, where the model with UV shielding has more abundant water. In the case of CO, there is a clear depletion in abundance for the model with water UV shielding. On the disk surface, CO can be destroyed by two pathways. In the highest reaches where CO is not fully shielded, it can be directly dissociated by UV radiation or through reactions with He⁺, which requires ionization by X-rays. Both pathways create free oxygen which is stolen to make H₂O before CO can re-form. In summary, the important reaction pathway is

$$\begin{eqnarray}&&\begin{array}{l}\mathrm{CO}+h\nu \to {\rm{C}}+{\rm{O}}\\ {\mathrm{He}}^{+}+\mathrm{CO}\to {{\rm{C}}}^{+}+{\rm{O}}+\mathrm{He}\\ {\rm{O}}+{{\rm{H}}}_{2}\to \mathrm{OH}+{\rm{H}}\\ \mathrm{OH}+{{\rm{H}}}_{2}\to {{\rm{H}}}_{2}{\rm{O}}+{\rm{H}},\end{array}\end{eqnarray} \tag{ 1 }$$

which leaves carbon without any oxygen to re-form CO. This excess carbon is sequestered in large (hydro)carbon-chain species, which will be discussed in Section 4.1.

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The behavior of CO₂ varies with radius. In the inner 1 au, the progressively lower temperature at increasing radius increases the CO₂ formation rate relative to the water formation rate (e.g., Bosman et al. 2018). Outside of the CO₂ midplane ice line, the temperatures of the surface layers become too low for the efficient formation of OH, the main precursor of CO₂, causing a strong CO₂ abundance drop. H₂O, C₂H₂, HCN, NO, CH₄, CH₃CN, and NH₃ all experience a similar temperature-driven drop in abundance around 0.9 au.

We have selected a number of species that have many orders of magnitude difference in abundance, between the standard model and including water UV shielding, out to the CO₂ ice line: C₂H₂, HCN, CH₄, CH₃CN, and NH₃. For these species, water UV shielding has a significant effect, enhancing these abundances by >3 orders of magnitude, in disks with high gas-to-dust mass ratios, where there is significant dust settling and growth. We expect that the species with an enhanced abundance due to water UV shielding will be able to be observed more prominently at heights farther up in the disk atmosphere. Figure 3 shows the emitting columns for these species at 0.3 and 0.6 au along with the other abundant species that are present in our UV-shielded model. It can be seen that the impact of UV shielding on molecular abundance yields significant results, impacting abundance at radii less than 0.9 au.

Figure 2 shows the 2D abundance distributions for the standard model and the water-shielding model over the inner 5 au for C₂H₂, CH₄, NO, HCN, CH₃CN, and NH₃. Looking at the IR-emitting layer, the area above the red line at z/r ∼ 0.15, it is clear that this region is most affected by the inclusion of water UV shielding. We see that C₂H₂, CH₄, CH₃CN, and NH₃ are found in the IR-emitting layer only if water UV shielding is included, while NO becomes depleted relative to the standard model, as was found in the average abundances in Figure 1. At deeper layers, below the IR-emitting layer, practically identical results are seen for both models, reflecting how the differences in abundance are limited to the surface layers. Thus, Figure 1 includes the entire vertical column of gas that is impacted by UV shielding.

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4.1. Carbon

In Figure 1, the CO abundance is reduced within the CO₂ ice line when water UV shielding is included. The self-shielding of water lowers the abundance of water photoproducts, such as atomic oxygen and OH, in the gas. These species are critical in the formation of CO, so its formation is slowed. This effect, combined with dissociation reactions with He⁺ in the upper atmosphere as discussed in Section 3, reduces the abundance of CO. Thus, more carbon is available for the formation of other species.

Similarly to CO, we can also see that the abundance of CO₂ is reduced when water self-shielding is included due to atomic oxygen and OH-poor environment (Bosman et al. 2022b); both are crucial to its formation. In contrast, we see increases in abundance for HCN, C₂H₂, CH₄, and CH₃CN.

Figure 3 shows that at 0.3 au, most of the carbon is incorporated into long carbon chains, such as C₆H₂ and C₉H₂ for the UV-shielding model with abundances of 3 × 10⁻⁶ (relative to total H). This result is similar to Wei et al. (2019), who explore releases of carbon from refractory carbon-rich grains. They find the efficient creation of large carbon-chain species if carbon grain destruction is included. Though there are differences in the origin of the carbon chains between Wei et al. (2019) and our models, the similar end results suggest that inner disk chemistry drives large carbon-chain production whenever there is little atomic oxygen available.

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Past models by Woitke et al. (2018) and Anderson et al. (2021) have found low abundances and fluxes of C₂H₂. In their models, matching observation and theory requires a high C/O ratio, thus creating an inability to reproduce H₂O observations. Figure 1 shows that the C₂H₂ abundance is elevated in an effectively oxygen-poor environment, as created by UV shielding in gas that is oxygen rich (i.e., stellar O/H in content). As T_gas > T_dust in this region, this should lead to a strong increase in 13.7 μm C₂H₂ emission. At 0.3 au, our models produce a C₂H₂ column density of 8.4 × 10¹⁷ cm⁻² in the region of the disk in the IR-emitting layer. The models of Anderson et al. (2021) have found the column density of C₂H₂ to be in the range of 10¹⁴−10¹⁶ cm⁻², and Woitke et al. (2018) found it to be equal to 10¹⁷ cm⁻² for a C/O ratio of 0.46. It should be noted that most of this column is at a low gas temperature of 230K and thus only weakly contributes to any line emission. Our models thus produce a larger amount of C₂H₂ in higher regions of the disk without invoking an elevated C/O ratio.

Compared to the observations of Salyk et al. (2011b), they find best-fit columns on the order of 10¹⁴−10¹⁵ cm⁻² for C₂H₂. This is low compared to our value of 10¹⁸ cm⁻² at 0.3 au. This discrepancy could be explained by the way the column is derived in Salyk et al. (2011b), in which the emitting area for water was determined and applied to all species. The region in which C₂H₂ has a high abundance is smaller than that of water, so it is expected that the C₂H₂-emitting region is smaller as well. This smaller emitting region would also imply a C₂H₂ excitation temperature that is higher than that of H₂O, as is seen in Salyk et al. (2011b). If the actual emitting region of C₂H₂ is smaller than assumed in the fitting by Salyk et al. (2011b), then the column will have to be decreased significantly to compensate and still produce the same total flux. This could explain the mismatch between the observation-derived columns and our predicted columns.

Finally, it is important to note the carbon species that we expect to see in this region. We likely expect to observe CH₄ with a column of 8 × 10¹⁵ cm⁻² at 0.3 au. High abundances of C₃ and long carbon chains with low hydrogenation are seen in Figure 3, which indicates that these species could have detectable band emission. It is unlikely to detect C₂H₆ and C₂H₄ in this region as the models produced columns of less than 6 × 10¹³ cm⁻² and 2 × 10¹⁴ cm⁻², respectively.

4.2. Nitrogen

Water UV shielding has a strong effect on the nitrogen-bearing species, increasing abundances of HCN, NH₃, and CH₃CN while lowering the abundance of NO. The abundance of HCN is known to be sensitive to the gas-phase C/O ratio (e.g., Cleeves et al. 2018). Our model effectively changes the C/O ratio and, thus, HCN rises in abundance for the water UV-shielding model. This is in large part driven by the chemistry discussed in Section 4.1. However, the changes in NH₃ and NO imply that the active nitrogen chemistry is also changed.

There seem to be three driving factors for the increased abundance of NH₃, HCN, and CH₃CN. The first driving factor is due to an impediment placed within the formation pathway of N₂. N₂ is primarily formed from atomic N by the reactions

$$\begin{eqnarray}&&\begin{array}{c}{\rm{N}}+{\rm{OH}}\to {\rm{NO}}+{\rm{H}}\\ {\rm{NO}}+{\rm{N}}\to {{\rm{N}}}_{2}+{\rm{O}}.\end{array}\end{eqnarray} \tag{ 2 }$$

With the inclusion of water self-shielding, the OH abundance is lowered, and this channel is suppressed. Further, in both the standard and full models, N₂ formation through CN + N →N₂ + C is suppressed by the competition with the CN + H₂ → HCN + H reaction. The slow N₂ formation leads to more nitrogen being available for species beyond N₂, most notably HCN and NH₃. This also impacts the abundance of NO, which is formed less efficiently in the UV-shielding model and thus has a lower abundance (Figure 1).

NH₃ formation is initiated by the reaction of He⁺ with N₂ or HCN, forming N⁺. The addition reaction with H₂ allows for the eventual formation of NH₄ ⁺, the precursor to NH₃. The main destruction channel for NH₃ in these hot layers is atomic H. Suppressing the photodissociation of H₂O lowers the production, and thus the abundance of atomic H. This increases the NH₃ lifetime and thus abundance as more hydrogen is available in the form of H₂. We see that NH₃ has a maximum abundance occurring at the water-ice line with the inclusion of UV shielding of roughly 10⁻⁷, a value four orders of magnitude higher than when UV shielding is excluded.

Lastly, the active carbon chemistry allows for atomic nitrogen to react with the abundant carbon chains (C_xH, C_yN), which produces CN, which reacts with H₂ to form HCN. HCN can then react with the more abundant CH₃ ⁺ to form CH₃CNH⁺, the precursor for CH₃CN.

The only nitrogen species that has so far been observed in the inner disk is HCN. HCN has been observed with columns of 10¹⁴−10¹⁵ cm⁻² in Salyk et al. (2011b) and predicted to have columns of 10¹⁴−10¹⁶ cm⁻² in the line-forming region at 0.3 au by Woitke et al. (2018). Though the column by Woitke et al. (2018) matches the observation, their emission line is weaker, similar to the case of C₂H₂ as the column is built up within deeper, cooler gas. Higher HCN fluxes are only seen by letting the C/O ratio approach unity in Woitke et al. (2018). We produced a column of 10¹⁹ cm⁻² with the effects of UV shielding included. This column has a strong contribution from warm surface layers (e.g., Figure 2 and thus the flux from our model is likely stronger than that with the C/O = 0.46 from Woitke et al. (2018).

The species that might be detectable are NO and NH₃ with columns of 5 × 10¹⁶ cm⁻² and 3 × 10¹⁶ cm⁻², respectively, while it is less likely to observe CH₃CN with a column of 9 × 10¹⁴ cm⁻².

In this work, we have studied the effects of water UV shielding on the chemical compositions of the inner, planet-forming region of protoplanetary disks. Specifically, we are looking at chemical abundances in the upper disk atmosphere where the IR line emission originates and the effects of water UV shielding are most prominent. This is done in order to further our understanding of observed emission from organics, which will be critical for the interpretation of observations by the James Webb Space Telescope.

We have concluded that water self-shielding has notable effects for hydrocarbons and nitriles. The lack of OH produced by H₂O dissociation suppresses N₂, CO, and CO₂ formation. As a result, there is more carbon and nitrogen available for a rich carbon and nitrogen chemistry (>3 orders of magnitude), boosting the abundance of C₂H₂, CH₄, HCN, CH₃CN, and NH₃.

The depletion seen in CO and CO₂ cannot be explained alone by the formation of species such as HCN, C₂H₂, CH₄, and CH₃CN. A significant amount (up to 53%) of the total volatile carbon finds its way into larger carbon chains, such as C₆H₂ and C₉H₂. The nitrogen released from N₂ finds its way to HCN with traces in CH₃CN and NH₃. We expect to observe CH₄, HCN, NO, and NH₃ with column densities shown in Figure 3.

The inclusion of water UV shielding provides a way to increase the production of C₂H₂ and HCN, which have been historically underproduced, without invoking a C/O ratio near unity. Model abundances for H₂O and CO₂ are already in agreement with observations, noted in Paper I and Paper III, thus this is a step forward in matching all four species at the same time.

Water UV shielding is important for the entire chemical inventory. Through its ability to both block UV rays from penetrating deep into the disk and to create an effectively oxygen-poor environment, formation conditions become more favorable for various hydrocarbons and nitriles. This work has shown that water UV shielding impacts the inner disk chemical composition and better reproduces observation. Thus, further studies that vary the C/O ratio with UV shielding are needed to advance our understanding of the inner disk chemistry and its evolution.

S.E.D, A.D.B. and E.A.B. acknowledge support from NSF Grant#1907653 and NASA grant XRP 80NSSC20K0259.

Software: astropy (Astropy Collaboration et al. 2013, 2018), SciPy (Virtanen et al. 2020), NumPy (van der Walt et al. 2011), Matplotlib (Hunter 2007).

Figure 4 shows the 2D structure of the gas temperature and density, dust density, gas-to-dust ratio, and UV and X-ray radiation fields for the model with water UV shielding. The UV radiation field is relative to the interstellar radiation field (Draine 1978). The red line at z/r ∼ 0.15 signifies the lower bound of the IR-emitting layer. Figure 5 compares the 2D abundance structure of H₂O and CO₂ for a full chemical network, as employed in this work, and a reduced chemical network, used in Bosman et al. (2022a). We can see that for both the standard model and model with water UV shielding, the resulting abundances in our estimation of the IR-emitting layer for both species are independent of the chemical network used.

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